I want to sequence several samples of mRNA paired end 2x150 and for each sample I want 100 million reads (100 million for forward and 100 million for reverse)

the supplier proposes to sequence 3 samples per lane of flow cell of the Hiseq 4000 technology.
So what I understood 3 samples x 200 million reads (100 million for forward and 100 million for reverse) = 600 million reads per lane

but when I looked for the performances Illumina Hiseq 4000 I found this technology can generate 312000000 Clusters per lane only. How can this technology generate 600 million reads per lane ??

What is sequenced is a so called "fragment". To simplify, they will sequence each sample with 100 mln fragments, and each fragment is sequenced from both ends, i.e. paired end sequencing. And for each fragment you will have two reads - forward and reverse. So what the provider tells you is correct. Anyway, always ask them for details and explanations if you have doubts to avoid potential misunderstandings and waste of resources.